An organic electroluminescent element (hereinafter, also referred to as an organic EL element) has a structure including a light emitting layer, which is composed of a light emitting compound, and a cathode and an anode which sandwich the light emitting layer. The organic EL element emits light (fluorescence or phosphorescence) generated as a result of deactivation of excitons formed by the recombination of holes and electrons injected in the light emitting layer from the anode and the cathode, respectively, in an applied electric field. The organic EL element is an entire solid element including a layer of organic material having a thickness of several sub-microns between the electrodes, and can emit light at a voltage of only several to several tens volts. Accordingly, the organic EL element is expected to be used in next-generation flat displays or illuminations.
On development of organic EL elements for practical use, an organic EL element involving phosphorescence emission from an excited triplet state was reported by Princeton University (refer to, for example, Non-Patent Literature 1). Studies on materials exhibiting phosphorescence at room temperature have become more active since then (refer to, for example, Patent Literature 1 and Non-Patent Literature 2).
Furthermore, an organic EL element involving recently discovered phosphorescence emission has a light emitting efficiency, in principle, about four times higher than that of the element involving conventional fluorescence emission, leading to not only development of materials but also worldwide research and development on layer structures and electrodes for the organic EL element. For example, many compounds, primarily heavy metal complexes, such as iridium complexes, have been synthesized and investigated (refer to, for example, Non-Patent Literature 3).
Although such system has significantly high potential, quite unlike the organic EL devices utilizing fluorescence emission, the organic EL devices utilizing phosphorescence emission has an important technical problem which is to control the position of the emission center, in particular to achieve recombination and stable light-emission inside the light emitting layer, from the viewpoint of efficiency and lifetime of the elements.
Nowadays, there are well known multilayer elements each including a hole transport layer (located on an anode side of a light emitting layer) and an electron transport layer (located on an cathode side of the light emitting layer) which adjoin the light emitting layer (refer to, for example, Patent Literature 2). Mixture layers using host compounds and phosphorescence emitting compounds as dopants have been widely used for light emitting layers.
Requirements for application of organic EL elements to displays and illuminations are highly stable chromaticity for environmental temperature in the case that the organic EL element continuously emits light for long hours, the case that the organic EL element is under high temperature and high humidity, or other cases. In applications to illumination light sources, the demand for the stability of the emission color is especially severe. Accordingly, it is an important issue how to ensure stable chromaticity in order to put the organic EL element into practice in the illumination light sources.
Metal complexes having specific ligands have been recently disclosed as blue phosphorescence emitting compounds having high potentiality in Patent Literature 3. Patent Literature 3 also discloses dibenzofuran and dibenzothiophene derivatives as host compounds which are used together with blue phosphorescence emitting compounds and are combined with conventional hole transport layers and electron transport materials. Unfortunately, the organic EL element composed of metal complexes as described in Patent Literature 3 is still insufficient in light emitting efficiency, lifetime and stable chromaticity.
For example, Patent Literature 4 discloses a method of providing an element having high efficiency achieved by electron blocking of a hole transport layer composed of laminated hole transport sublayers, of which the hole transport sublayer closest to the light emitting layer contains a carbazole derivative, dibenzofuran derivative, dibenzothiophene derivative, or fluorene derivative of triphenylamine.
Patent Literature 5 discloses a method for reducing the degree of hole injection to a light emitting layer by defining the structure, the mobility, and the HOMO level of the hole injection layer to provide an element with high efficiency and long lifetime.